Bone fasteners are typically bone screws or pegs which are inserted into bone. Basically, bone screws are used in two different ways: in a first application bone screws serve to fix bone or bone fragments in a desired position relative to one another. In this case the bone screw is used alone. In a second application the bone screw is used as a compression screw or a locking screw in order to position additional elements as fixation elements in or on bone. Here, bone screws are used, for example, together with intramedullary nails (marrow nails). Another area of application is osteosynthesis, in which a biocompatible element substitutes for a bone or a bone fragment. For example, a bone plate can be anchored by bone screws to bone.
Bone screws are available in a plurality of variations for different applications. For example, U.S. Pat. No. 6,030,162 A relates to a compression bone screw for creating an axial tension along its length. The screw has a screw thread formed thereon in a leading section and a trailing section with a threaded or an unthreaded section therebetween. The core diameter as well as the outside diameter of the thread varies over the entire length of the bone screw.
U.S. Pat. No. 6,355,043 B1 relates to a bone screw for anchoring a marrow nail. The bone screw includes a head part, a middle part, and a distal part. A thread extends from a proximal portion of the head part to the distal part, at a constant core diameter over the entire length. The proximal portion of the head part includes a proximal thread having an outer diameter which is greater than the outer diameter of the middle and distal part and greater than an inner diameter of a transverse bore in the marrow nail.
U.S. Pat. No. 4,875,475 and EP 1 260 188 A1 relate to a bone screw for insertion into bores of an intramedullary nail. The bone screw has a head and a shaft attached thereto. The shaft includes a front region and a rear region. The front region has a thread for anchoring the intramedullary nail to bone. The rear region of the shaft is unthreaded and has a smooth outer peripheral surface. The smooth rear region of the shaft is positioned within a transverse bore in the intramedullary nail for supporting the nail.
The conventional bone screws for anchoring an intramedullary nail to bone have several drawbacks. A bone screw with a continuous thread along its entire shaft has a low endurance limit. Since such a bone screw has a low strength, a greater diameter is often desirable. Moreover, a bone screw with a shaft having a thread over the full length of the shaft can only be used to fix an intramedullary nail to bone. However, such a bone screw may not be appropriate to interact with the intramedullary nail for compressing aligned bone pieces. Typically, bone screws for supporting an intramedullary nail include a shaft having a threaded front region and an unthreaded smooth rear region. Once the intramedullary nail is implanted, the smooth shaft of such bone screws is arranged within a transverse bore of the intramedullary nail. The side wall edges of the transverse bore in the nail or an adjusting means abuts against the smooth shaft region of the bone screw for supporting the nail and adjusting force to act between aligned bone pieces for holding them together. However, bone screws with a smooth unthreaded shaft portion can only be removed (explanted) by a self-retaining screw driver. Moreover, due to the smooth shaft surface, such screws can get stuck within the transverse bore of the intramedullary nail or within the bone, whereby the explantation is extremely hindered.